Something that's aglow is gleaming with light. You might sit outside at night to see the full moon, aglow in the dark sky.
The adjective aglow is a poetic, slightly old-fashioned way to say "glowing" or "radiant." Some things are literally aglow, like the sunshine glinting off the surface of a lake, and others are figuratively aglow, like your sister's smiling face, aglow with health and happiness. The poet Samuel Taylor Coleridge is credited with the first use of aglow, in his 1817 "Sonnet to William Wordsworth."